Inverted cross field device having an arcuately segmented cathode



July 29, 1969 Filed May 5, 1966 H. PETERS. JR

INVERTED CROSS FIELD DEVICE HAVING AN ARCUATELY SEGMENTED CATHODE July 29, 1969 P. H. PETERS. JR 3,458,754

INVERTED CROSS FIELD DEVICE HAVING AN ARCUATELY SEGMENTED CATHODE Filed May 3. 1966 2 Sheets-Sheet 2 F/g 4 .3 F/g. 5. m

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United States* Patent O1 3,458,754 Patented July 29, 1969 ice U.S. Cl. S15-39.3 26 Claims ABSTRACT OF THE DISCLOSURE An inverted cross field device is described having a cylindrical anode formed by an array of alternating bars and slots encompassed by a segmented cathode to form an annular interaction space therebetween to which crossed electric and magnetic elds are applied. The arcuate sections of the cathode are electrically interconnected to permit direct current flow between cathode sections with R.F. current ow between the sections being inhibited by mounting the sections on a lossy dielectric substrate and interconnecting the cathode leads extending through the dielectric substrate with small diameter conductors. Dual annularly shaped straps are positioned at the approximate center of the anode bars to electrically interconnect alternate bars and energy is coupled into the anode cavity through conductive cylinders joining the respective straps with top and bottom plates mounted on opposite ends of the anode Ibars. The cross eld device is particularly suitable as an electron beam accelerator wherein a high current electron beam introduced through suitable apertures into the center of the anode cavity is accelerated by a voltage at least an order of magnitude higher than the voltage induced at the anode bars.

The present invention relates to an improved crossedfield device having a novel structure.

In general, crossed-field devices such as magnetrons comprise an anode, having vanes or bars separated by spaces, and a cathode juxtaposed to form an annular interaction space. Crossed electric and magnetic fields are applied, a rotating space charge cloud is established and, through interaction with the anode, the space charge is bunched. The rotating high density bunches excite an R.F. electromagnetic wave on the anode. -Eifective transfer of the D.C. input energy from the electrons to the electromagnetic wave is achieved when the electromagnetic wave is propagated uniformly about the magnetron and is in synchronism with the space charge; that is, when the two travel at the same angular velocity about the magnetron anode.

The purpose of this invention is to provide a crossedeld device having a structure which is advantageous in several respects over presently known devices. For example, crossed-field devices constructed in accordance with the present invention produce increased power at a given operating frequency, are more readily controlled and are less expensive to operate. Furthermore, if desired, the crossed-field device of the present invention can be a-rranged for voltage step-up from the RF. voltage in the anode bars by the space charge to a point in the anode structure at which the useful load is connected.

Accordingly, it is an object of the present invention to provide a crossed-field device having a novel structure.

A further object of the present invention is the provision of a new and improved high power crossed-field device.

A further object of this invention is the provision of a stable high power crossed-field device.

Another object of the present invention is the provision of a crossed-field device which produces voltage step-up from input to output.

A particular object of this invention is the provision of a crossed-field device wherein the useful load can be positioned at the point of maximum output voltage.

Briefly, in accord with one embodiment of this invention, I provide a new and improved crossed-field device including an anode and a cathode which define an annular interaction space. The device is of the inverted type whe-rein the anode is mounted within the surrounding cathode. In accord with this invention, the cathode comprises a plurality of spaced segments, the cathode segments being interconnected to maintain a constant D.C. potential. Damping means are associated with the interconnection means to substantially eliminate the passage of RF. current between the cathode segments. The anode comprises a plurality of spaced bars disposed in a cylindrical array. Each of the anode bars is connected both to a top plate and a bottom plate, the plates and the barstogether defining an anode cavity, and means are provided for coupling energy developed in the anode to a load.

In a specific embodiment of this invention, the cathode surfaces comprise a secondary emission material and one or more sources of primary electrons are provided to initiate secondary emission from the cathode to make up the space charge of the device. In accord with another specific feature of this invention, the anode may be adapted for inclusion of a load at the center of the anode cavity so that the output power is coupled to the load at the point of maximum output voltage.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by refe-rence to the following description taken in connection with the appended drawings in which:

FIGURE 1 is a vertical cross section view of a crossedeld device in accord with the present invention;

FIGURE 2 is a perspective view of the cathode shown in FIGURE 1;

FIGURE 3 is a perspective view, partially broken away, of the anode shown in FIGURE l;

FIGURE 4 is a top plan view of an alternative anode constructed in accord with the present invention;

FIGURE 5 is a vertical cross-section view of an electron beam accelerator constructed in accordance with this invention; and

FIGURE 6 is a vertical cross-sectional View of a cascaded series of the accelerators shown in FIGURE 5.

In FIGURE 1, a crossed-field device 10 in accord with the present invention is illustrated. The device includes an anode 11 and a cathode 12 defining therebetween an annular interaction space 13. The enclosure of the space 13 is completed by end hats 14 which prevent the escape of electrons. The device 10 is of the inverted type wherein the anode 11 is mounted within the cathode 12. Both are disposed in a conductive hermetically sealed enclosure comprising conductive cover plates 15 and 16 and cylinder 17. Alternatively, cylinder 17 may comprise a ring of high loss dielectric material with a sputtered metal coating. In accord with the well-known operation of such devices, cross fields including a D.C. electric eld of about 10-15 kilovolts between the cathode and anode and a magnetic eld of about 15010-2000 gauss, are applied to the space 13. The magnetic eld is illustrated in FIGURE 1 'by the arrows H. Interaction of these fields with electrons from the cathode produces a rotating bunched space charge within the interaction space 13. The rotating space charge excites a radio frequency electromagnetic wave on the anode 11 which may be utilized, for example, by coupling the power out through loop 18 to coaxial line 19 which is then connected to a load. Alternatively an electric-held probe can be used ,if desited.

I have found that re-entrant R.F. electromagnetic waves can ariseo n the cathode of this type of inverted crossed-field device at frequencies for which the mean circumference of the annular interaction space is an integral multiple of the wavelength. If any one of these frequencies are lower than the desired operating frequency, the device tends to excite a wave at this frequency on the cathode rather than exciting the anode wave. This type of interaction results in the loss of input power which is dissipated in the form of electron heating of the cathode. Thus, if the size of the device is large enough to support a cathode wave of lower frequency (longer wavelength) than the operating frequency, the device will not operate efiiciently. Therefore, I have found that previously known inverted crossed-field devices are limited in circumference by the wavelength of the operating frequency; that is, the mean circumference of the interaction space cannot be greater than the operating wavelength. Since the power output of crossed-field devices depends upon the size, the power obtainable at a given frequency is also limited.

In accord with my invention, this problem may be overcome by segmenting the cathode to prevent excitation of the unwanted waves on the cathode. Excitation can still occur through the electrical connections required to maintain the cathode segments at equal D.C. potentials; I have discovered that this maybe prevented by providing damping means associated with the electrical connections to prevent excitation of the undesired waves.

The cathode 12 of FIGURE 1, shown more clearly in FIGURE 2, is constructed in accordance with this concept. In the particular embodiment illustrated, the cathode comprises a plurality of segments 20 mounted on a surrounding block of dielectric material 21 supported on pedestals 22 which are affixed to cover plates. The segments are electrically interconnected to maintain equal D.C. potentials by conductive means such as conductive rods 23, mounted in the segments, and wires 24 which connect the rods 23. The cathode segments 20` are conductive and preferably comprise aluminum. In this case, to prevent heat damage to the rod mounting, a stainless steel boss 25 is force-fitted into a hole in the aluminum segment 20 and the rods 23 are screwed into the boss. At the exterior of the insulating block 21, the rods are threaded to provide for mounting the cathode segment 20 to the insulating block 21 by washer 26 and nut 27. The rods are brought out through the cylinder 17 and supported by insulators 28.

The insulating block 21 comprises a lossy dielectric such as slicon carbide. The rods 23 are brought out through very closely fitted holes in the silicon carbide so that, if an R.F. current passes through the rod, the field associated with it is coupled into the silicon carbide block. The high loss dielectric damps such currents very effectively and accoringly, the R.F. current is reduced or eliminated. In further accord with this invention, the wires 24 used to interconnect the rods 23 are preferably very small in diameter so that they act as chokes to further damp any remaining R.F. current. Accordingly, although the cathode segments are completely connected to present a constant D C. potential to the anode, any R.F. mode which arises in the cathode is damped and cannot be excited.

Thus, although the mean circumference of the interaction space is larger than the operating wavelength, the excitation of lower frequency (higher wavelength) cathode waves is prevented by the division of the cathode into the segments 20. The damping means associated with the necessary D.C. connections removes substantially all of the energy of any R.F. current which attempts to pass through the D.C. connections as well as any which may arise in the capacitance which exists between the cathode segments. Other damping means may also be used without departing Afrom the spirit o f this invention,

In general, the surface of cathode segments 20 may be of any type suitable for the emission of electrons to form the space charge of the device. However, the hot cathodes conventionally used are expensive to build and operate and the secondary emission cathodes which have been proposed, which depend on field emission from the cathode to initiate operation, require additional means and power for initiating the discharge. Therefore, an important feature of the present invention is the provision of an improved means for producing the space charge of the crossed-field device.

As shown in FIGURE 1, this means comprises a source of primary electrons which are caused to impinge on the cathode surface to cause secondary emission. Specifically, the cathode 12 includes at least one thermionic electron emitter filament 29 mounted within and approximately flush with the inner surface of a cathode segment 20. Tube 30 is used in place of rod 23 and a ceramic rod 31 is fitted therein. Ceramic rod 31 has two holes through it for the passage of the heater wires 32 which are connected across a source of heating power 33. Conveniently, one heater wire may be connected to the negative terminal of a D.C. power supply 34. The positive terminal of the power supply is connected to the anode and, preferably, grounded. The power supply produces the electric field necessary for crossed-field interaction. When the field is applied to the cathode, secondary emission electrons are emitted from the surface 35 of cathode segments 20` due to impact of the primary electrons from the filament. These produce further emission and a space charge is formed in the device. I have found that the bombardment of the cathode by such primary electrons produces a sufficient quantity of secondary electrons to form a dense space charge of high current. Thus, the necessity of providing a heater and the expense of heating the entire cathode surface can be avoided. Also, the complexities of starting by a 4field emission process can be avoided.

A further advantage of this construction is the ease of controlling device operation. For example, the DC. voltage source may include a pulsing circuit for applying the negative voltage to the cathode during pulses of predetermined length. When the D.C. voltage is applied, secondary electrons become available immediately to form the space charge. Therefore, due to the provision of the emitting filament, operation during each pulse begins typically in a time of 10 to 100 nanoseconds. This operation is even more easily achieved by an alternative arrangement wherein the filament 29 is not connected to the cathode, but is supplied from an isolated circuit comprising an A C. heater supply and is pulsed by a source of low D.C. voltage, for example, a few hundred volts, to inject electrons into the interaction space 13. In this embodiment, pulsing of the high anode-cathode voltage is avoided. When the filament is pulsed, the injection of primary electrons produces secondary emission to form the space charge. When the control pulse ends, primary electrons are no longer injected and the space charge collapses. Thus, R.F. output pulses are produced at the rate at which pulses are applied to the primary emitter without the necessity of pulsing the main D.C. supply.

The anode of the device shown in FIGURE l, illustrated more clearly in FIGURE 3, is also a significant aspect of the present invention. The anode 11 comprises upper and lower plates 36 and 37 between which are connected a plurality of spaced anode bars 38. As the rotating space charge passes the alternating bars and spaces of the anode, an RF. electromagnetic wave is excited on the anode. In a manner similar to conventional techniques, the alternate anode bars are connected by straps 39 and 40 to ensure operation of the anode in the pi-mode. The structure of this invention also includes conductive cylinders 41 and 42 which connect straps 39 and 40, respectively, to the upper and lower plates 36 and 37.

The anode comprises a re-entrant, strapped, delay line having a backward-Wave phase-frequency characteristic.

The lowest resonant frequency of the anode delay line corresponds to a phase shift between bars of pi radians. As the space charge rotates around the annular interaction space 13, an R.F. voltage is established along the length of each bar. The R.F. Voltage on one bar at one instant of time is illustrated by the curve ERF in FIGURE 1. It is a characteristic of this delay line operating in the pi-mode that the ends of the bars remain at zero R.F. voltage and the potentials of the respective sets of alternate bars oscillate in opposite directions.

Due to the alternate connection of the respective straps 39 and 40 to the anode bars, the interbar voltage appears between the straps and R.F. currents, represented by arrows I1 and I2, are produced in the cylinders. The anode cavity and the anode bars form a coupled circuit which is resonant at the operating frequency and the currents I1 and I2 couple energy from the array of bars into the cavity at this frequency in the T M01 circular mode. Accordingly, an R.F. electric field is set up between the top and bottom of the cavity which has a maximum along the axis of the device. This operation is particularly advantageous in applications where a load can be positioned within the anode cavity since the maximum R.F. voltage appears in the center of the cavity and can be applied directly to the load. Because the cavity is resonant, the voltage maximum at the center is higher than the maximum R.F. voltage on the anode segments and, in fact, may be larger by one to two orders of magnitude. Thus, a large step-up in voltage can be achieved. The high voltage may be applied to a load, for example, by mechanically isolating the atmosphere in the cavity from the interaction space and providing a removable portion in either the top or bottom plate 36 or 37 through which a load may be inserted within the cavity. In other applications, the output of the device may be applied to an external load by means of the previously mentioned loop 18 which couples the R.F. field on the anode to the coaxial cable 19. The loop 18 is positioned at an appropriate radial position so that the impedances of the cable and of the cavity at that point are matched.

A particular advantage of the anode structure described arises from the fact that the R.F. waves induced in all of the anode segments are coupled into a common cavity. This effect produces an extremely frequency-stable device since unavoidable dimensional variations from bar to bar result in negligibly small variations in the inter-bar voltages. This is because of the large value of the stored energy in the central cavity. A second reason for good frequency stability observed in this device is that variations induced by the load reactance, known as pulling, operate against the same large quantity of stored energy in the cavity and the effect thereof is reduced. Another advantage of this structure is that a single tuning element, for example, a screw-tuner 44 at the center of the cavity is effective in simultaneously tuning the entire bar array to a particular frequency. Thus, the large complex tuning elements often required in distributed magnetron anodes are avoided.

In the anode illustrated in FIGURES 1 and 3, the coupling of the generated wave into the anode cavity is done by means of the current owing in cylinders 41 and 42. FIGURE 4 illustrates an alternative anode wherein cylinder 41 is replaced by a plurality of individual conductive members such as posts 43. The cylinder 42 is also replaced by similar posts not shown in the view. Each post 43 is connected between the strap 39 and the anode plate 36 and the posts replacing cylinders 42 are connected between the strap 40 and anode plate 37. The currents indicated by the arrows I1 and I2 in FIGURE 1 are developed in these conductors in the same manner as described in connection with the cylinders 41 and 42. Each post produces a magnetic field around itself as illustrated by the lines of magnetic force HRF in FIG- URE 4. In this embodiment, the coupling to the cavity is basically magnetic rather than electric which may be desirable for some applications where a higher step-up is desired.

A particularly important and improved embodiment of the present invention is shown in FIGURE 5 and comprises an electron beam accelerator which utilizes the crossed-field device described in connection with FIG- URES 1-3. The anode of FIGURE 4 might also be used if desired. In this device, an electron beam to be accelerated is developed in any convenient source such as an electron gun 50. The beam is injected through a pair of electrodes 51, 52 and a drift tube 53 wherein the beam separates into discrete bunches of electrons. The separation is accomplished by connecting the electrode 51 directly to the anode and by coupling the electrode 52 to the RF. field in the cavity by means of field probe 54.

Within the anode cavity, a pair of cylindrical re-entrant electrodes 55 and 56 are provided to define a gap 57 within which the electron beam is accelerated by the RF. field. After being accelerated by the voltage across the beam gap 57, the beam leaves the cavity region and enters an R.F. field free hood 58. This may be terminated in a thin metal window 59 which isolates the vacuum Within the accelerator from surrounding atmosphere while permitting passage of the beam. The hood 58 may include means for scanning or defocussing the beam if desired or, if the beam is too low in energy to pass through a metal window, a series of small differentially pumped apertures may be provided.

The feedback coupling arrangement provided by probe S4 simplifies the problem of synchronizing the bunched beam with the R.F. eld across gap 57 to achieve acceleration. The drift tube and re-entrant electrode 55 are of an appropriate length so that an electron bunch reaches the gap at or near the peak of the R.F. voltage.

In addition to the advantages previously described for crossed-field devices in general when constructed in accord with this invention, the electron beam accelerator shown in FIGURE 5 is particularly advantageous for several reasons. First, it is noted that the load or electron beam is provided at the center of the anode cavity. This permits application of the maximum generated voltage to the beam since the maximum voltage step-up occurs in the resonant cavity, between the cavity Wall and the center of the cavity. Thus, for a given voltage induced at the anode bars, the beam can be accelerated by a voltage at least an order of magnitude greater. Also, because the beam or load is within the cavity, losses which occur in other systems such as coaxial cables and in coupling loops do not occur in this device. The efficiency of application of the R.F. power to the beam at the gap is very high, on the order of Another advantage of this accelerator is the simplicity of synchronizing the electron bunches with the R.F. field by means of the field probe 54 connected to the beam bunching electrode 52. Also, even if the beam density is sufficiently high so that it changes the operating frequency of the device, the bunching rate of the electron beam is adjusted automatically so that the bunches continue to arrive at the gap at the right moment. This self-synchronizing feature avoids the complex circuitry required to accomplish a similar object in other accelerators. Also because of this feature, beams of high current can be accelerated.

The electron beam accelerator shown in FIGURE 5 is a compact unitary structure which is entirely of rugged construction. The integration of the power source and the accelerator reduce the size and Weight as well as the complexity of the structure. The strength and unity of construction eliminate the handling diiculties which are encountered with other more complex or more delicate systems. It is also noted that the entire outer structure of this accelerator is at D.C. -ground potential thus reducing the problems of voltage breakdown and of cooling. FIGURE 6 illustrates a further development of the electron beam accelerator of FIGURE 5 wherein a plurality of cascaded cavities, each driven by an inverted crossed-field device are used. The electron beam from the gun 50 is synchronized by bunching electrodes 51 and 52 and applied to a first accelerator 60. This accelerator is identical to that shown in FIGURE and operates in the same manner except that the accelerated beam is passed on to the next accelerator 61.

The accelerator 61 is similar to that described in connection with FIGURE 5 except that, in accord with this invention, any initiation of the space charge is accomplished by coupling R.F. energy from the device 60 through coupling elements such as loops 62 and 63. This energy in the cavity of device 61 causes eld emission from the cathode and the primary emitters described in FIGURE 1 can be omitted as is illustrated by the broken away portions of the devices 60 and 61. The cathode of device 60 includes a primary emitter 29 While that of device 61 simply comprises a secondary emission cathode which is initiated and sustained by the R.F. energy coupled from accelerator 60.

In the illustration of FIGURE 6, the electron beam is now passed through a window 59 and applied, for example to irradiate a target or in the production of X-rays. Of course, any number of the single-cavity accelerators can be stacked to produce a desired power and energy level.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A crossed-field device comprising:

(a) an anode; a cathode surrounding said anode and defining therewith an annular interaction space; and means for producing a rotating space charge in said space upon the application of crossed electric and magnetic fields thereto;

(b) said anode comprising a cylindrical array of alternating bars and slots forming a re-entrant delay line; a pair of conductive members respectively interconnecting opposite ends of said bars; and strapping means interconnecting alternate sets of said bars adjacent the mid-point thereof;

(c) said cathode comprising a plurality of arcuate segments forming an annular body juxtaposed with said array, conductive means interconnecting arcuately adjacent segments of said cathode and decoupling means associated with said conductive means to reduce R.F. transmission between segments.

2. A crossed-field device as claimed in claim 1 wherein said anode comprises a delay line having a backward wave phase-frequency characteristic.

3. A crossed-field device as claimed in claim 1 wherein said anode comprises a resonant cavity enclosed by said array and by a pair of conductive plates, each of said bars being connected to both said plates.

4. A cross-field device as claimed in claim 3 wherein said strapping means include two annular conductors interconnecting alternate sets of said bars adjacent the midpoint thereof and further including conductive means connected between each of said strapping means and the conductive plate proximate said strapping means for coupling energy from said array into the center of said anode.

5. A crossed-field device as claimed in claim 4 wherein said conductive means comprises a rod connected between the conductive plate and said strapping means for coupling energy into said anode.

6. A crossed-field device as claimed in claim 4 wherein said conductive means comprises a cylindrical member connected between the conductive plate and said strapping means for coupling energy into said anode.

7. A crossed-field device as claimed in claim 6 wherein said anode includes a pair of conductive plates deiining a resonant cavity; said strapping means comprises a pair of coaxial members, each of said members having extensions contacting alternate ones of said anode bars; and said conductive means comprises a pair of cylinders, each of said cylinders being connected between one of said plates and one of staid strapping members.

8. A crossed-field device as claimed in claim 1 wherein said device includes means for coupling said R.F. output to a load.

9. A crossed-field device as claimed in claim 8 wherein said coupling means comprises a magnetic field-sensing loop disposed within said array and connected to a coaxial cable.

10. A crossed-field device as claimed in claim 8 wherein said coupling means comprises an electric field-sensing probe disposed within said array and connected to a c0- axial cable.

11. A crossed-held device as claimed in claim 8 wherein said coupling means comprises conductive means defining a resonant cavity for coupling energy to said load within said cavity.

12. A crossed-field device comprising:

(a) an anode; a cathode surrounding said anode and defining therewith an annular interaction space; and means for producing a rotating space charge in said space upon the application of crossed electric and magnetic fields thereto;

(b) said anode comprising a cylindrical array of bars forming a re-entrant delay line, and strapping means interconnecting alternate sets of said bars;

(c) said cathode comprising a plurality of arcuate seg ments presenting a cylindrical surface to said anode, conductive means interconnecting arcuately adjacent segments of said cathode and decoupling means associated with said conductive means to reduce R.F. transmission through said conductive means.

13. A crossed-field device as claimed in claim 12 wherein said cathode comprises a plurality of conductive segments the interior face of an annular ring of high-loss dielectric material and said conductive means include conductors passing through tightly fitted holes in said dielectric material to damp R.F. currents between said segments.

14. A crossed-held device as claimed in claim 13 wherein said decoupling means further comprises means presenting a high impedance to R.F. current flow between arcuately adjacent segments of said cathode while presenting a low resistance to D C. current tlow between said arcuately adjacent segments.

15. A crossed-held device as claimed in claim 14 wherein said impedance means comprises a fine Wire connected between said segments.

16. A crossed-field device as claimed in claim 12 wherein the surface of said segments comprises a material which emits large numbers of secondary electrons, and wherein said means for producing a space charge comprises at least one thermionic electron emitter.

17. A crossed-field device as claimed in claim 16 wherein said thermionic emitter is mounted within and approximately fiush with the internal surface of one of said cathode segments.

18. A crossed-field device for accelerating an electron beam comprising (a) a cylindrical anode; a cathode surrounding said anode and defining therewith an annular interaction space; and means for producing a rotating space charge in said space upon the application of crossed electric and magnetic elds thereto;

(b) said cathode comprising an annular body juxtaposed with said anode and including means for preventing the excitation of R.F. waves thereon;

(c) said anode comprising a cylindrical array of bars forming a re-entrant delay line; a pair of conductive members interconnecting the respective ends of said bars; strapping means interconnecting alternate sets of said bars; and means for coupling R.F. energy induced on said bar array into said cavity;

(d) an electron beam means for introducing said electron beam into said cavity and (e) means for coupling said R.F. energy to said electron beam passing through the center of said anode.

19. A crossed-field device as claimed in claim 18 wherein said means for introducing said electron beam into said cavity comprises an electron emitter mounted on one of said conductive members, and means for directing electrons emitted therefrom through said openings in said electrodes.

20. A crossed-field device as claimed in claim 19 including means for bunching emitted electrons and synchronizing the arrival of said electrons in said anode with the R.F. eld in said anode comprising an annular bunching electrode disposed between said electron emitter and said anode and means for coupling RF. energy from said anode to said 'bunching electrode.

21. A crossed-field device as claimed in claim 19 and further including at least one additional accelerator cornprising (a) a cylindrical anode; a cathode surrounding said anode and deiining therewith an annular interaction space; and means for producing a rotating space charge in said space upon the application of crossed electric and magnetic elds thereto;

(b) said cathode comprising an annular body juxtaposed with said anode and including means for preventing the excitation of R.F. Waves thereon;

(c) said anode comprising a cylindrical array of bars forming a re-entrant delay line; a pair of conductive members interconnecting the respective ends of said bars; strapping means interconnecting alternate sets of said bars; and means for coupling R.F. energy induced on said bar array into said cavity; and

(d) means for coupling said R.F. energy to an electron beam passing through the center of said anode,

22. A crossed-field device as claimed in claim 21 wherein said means for producing a space charge in said additional accelerator comprises coupling means for coupling energy from said cavity of said irst accelerator into said cavity of said additional accelerator to produce secondary emission from said cathode of said additional accelerator.

23. A cathode for a crossed-field device comprising an annular array of conductive segments arranged to present a cylindrical, substantially continuous internal surface; means supporting said segments comprising an annular ring of dielectric material secured to the face of said cathode remote from said interaction space; and conductive means interconnecting arcuately adjacent cathode segments, said conductive means passing through tightly fitted holes in said dielectric material for damping R.F. currents passing between said segments.

Z4. A cathode for a crossed-field device as claimed in claim 23 and including further R.F. decoupling means comprising means in said conductive means presenting a high impedance to R.F. currents while presenting a low resistance to D.C. currents.

25. A crossed-field device comprising (a) a cylindrical anode; a cathode comprising an annular body surrounding said anode to define therewith an annular interaction space, said cathode including means for producing a rotating space charge in said space upon the application of crossed electric and magnetic elds thereto;

(b) said anode comprising a cylindrical array of bars forming a re-entrant delay line arranged for excitation of an R.F. wave thereon upon interaction with said rotating space charge;

(c) means for coupling R.F. energy induced on said array into the center of said anode;

(d) means for generating an electron beam; and

(e) means for introducing said electron beam through the center of said cylindrical array for applying said R.F. energy to said beam.

26. A crossed-held device as claimed in claim 2S including conductive members interconnecting the respective ends of said anode bars, at least one said conductive member having a central aperture therein, and means for directing electrons from said electron beams generating means through said aperture in said conductive member.

References Cited UNITED STATES PATENTS 2,454,337 11/1948 Okress 333-34 X 2,609,522 9/1952 Hull 315-3951 X 2,832,005 4/1958 Brown 313-338 X 3,096,462 7/1963 Feinstein 315-3975 X 3,305,751 2/1967 Brown 315-34 3,289,035 11/1966 Drexler S15-39.51

HERMAN KARL SAALBACH, Primary Examiner SAXFIELD CHATMON, IR., Assistant Examiner U.S. Cl. X.R. 

